1. Field of Invention
The invention relates to medical systems and methods and more particularly to an electroencephalograph (EEG) system and method having improved spatial resolution.
2. Related Art
Electroencephalography (EEG) is a noninvasive technique for monitoring brain function based on amplification of brain potentials recorded at the scalp. Although the EEG has been measured for over 60 years, its full potential as a technology for imaging brain function has not yet been realized. Its chief advantages, when compared with other brain imaging technologies, such as positron emission tomography (PET) are: (1) an unsurpassed millisecond-level temporal resolution necessary for resolving sub-second neurological processes; and (2) simplicity of the sensors (essentially a small piece of metal and an operational amplifier) which results in low cost and ability to record for prolonged periods of time in a naturalistic setting. The chief disadvantage of the traditional EEG method is poor spatial resolution. There are only two obstacles to substantially improving spatial resolution. The first is that more scalp recording sites are needed. While it is the usual clinical practice to record EEG from 19 sites, it has been demonstrated with recordings from up to 124 sites that additional spatial information is available. U.S. Pat. No. 4,736,751, incorporated by reference, describes a system using a larger number of electrodes and various digital computer based methods to obtain more information from the brain wave signals. U.S. Pat. Nos. 4,967,038, 5,038,782 and [patent to issue on U.S. Ser. No. 07/578,880], incorporated by reference, describe systems for conveniently obtaining EEG recordings from a larger number of electrodes and locating their positions. The second obstacle is that spatial enhancement procedures are needed to reduce blur distortion that occurs as electrical potentials generated in the brain at the micro-volt level are volume conducted through brain, cerebral spinal fluid, the low-conductivity skull, and scalp to the recording electrodes at the scalp surface. However, spatial enhancement is possible since the 3 dB point of the point spread function for conductance of potentials from the brain surface to the scalp averages about 2.5 cm and with 128 electrodes spaced evenly on an average adult head, the interlectrode distance is about 2.25 cm.
U.S. Pat. No. 4,416,288 to Walter Freeman entitled "Apparatus And Method For Reconstructing Subsurface Electrophysiological Patterns" discusses, in an EEG system, calculation of spatial deconvolution transformation of the detected electrode field potentials. However, no attempt is made to actually measure the shape and thickness of the scalp and skull of the subject, or the actual positions of the electrodes on the scalp, or to correct the potentials for distortion due to transmission through the skull and scalp.